1. The Field of the Invention
The present invention relates generally to the field of high speed optical communications systems. More particularly, embodiments of the invention relate to devices, systems, and methods for providing bi-directional multiplexed traffic on single optical fibers.
2. The Relevant Technology
Computer and data communications networks continue to develop and expand due to declining costs, improved performance of computer and networking equipment, the remarkable growth of the internet, and the resulting increased demand for communication bandwidth. Such increased demand is occurring both within and between metropolitan areas as well as within communications networks, such as wide area networks (“WANs”), metropolitan area networks (“WANs”), and local area networks (“LANs”). These networks allow increased productivity and utilization of distributed computers or stations through the sharing of resources, the transfer of voice and data, and the processing of voice, data, and related information at the most efficient locations.
Moreover, as organizations have recognized the economic benefits of using communications networks, network applications such as electronic mail, voice and data transfer, host access, and shared and distributed databases are increasingly used as a means to increase user productivity. This increased demand, together with the growing number of distributed computing resources, has resulted in a rapid expansion of the number of fiber optic systems required.
Through fiber optics, digital data in the form of light signals is formed by light emitting diodes or lasers and then propagated through a fiber optic cable. Such light signals allow for high data transmission rates and high bandwidth capabilities. Other advantages of using light signals for data transmission include their resistance to electromagnetic radiation that interferes with electrical signals; fiber optic cables' ability to prevent light signals from escaping, as can occur electrical signals in wire-based systems; and light signals' ability to be transmitted over great distances without the signal loss typically associated with electrical signals on copper wire.
Another advantage in using light as a transmission medium is that multiple wavelength components of light can be transmitted through a single communication path such as an optical fiber. This process is commonly referred to as wavelength division multiplexing (WDM), where the bandwidth of the communication medium is increased by the number of independent wavelength channels used. To accomplish wavelength division multiplexing, several specialized optical components are used, including demultiplexers (demuxes), multiplexers (muxes), mux/demux modules, and optical add/drop multiplexers (OADMs).
A demultiplexer generally takes as its input an optical transmission that includes a number of individual signals, with each signal being transmitted using a particular wavelength of light. An exemplary optical demultiplexer is shown in FIG. 1 and designated generally as 10. The optical demultiplexer 10 has an input port 12. The input port 12 receives a multiplexed transmission 14. In the present example, the multiplexed transmission 14 has four individual signals, each of different wavelengths, which are designated in this example as λ1, λ2, λ3, and λ4, as indicated in FIG. 1A. The optical demultiplexer 10 is a passive device, meaning that no external power or control is needed to operate the device. Although, in this example, the optical demultiplexer 10 is a passive device, it should be noted that active devices can be used in optical demultiplexing as well. Using a combination of passive components, such as thin-film three-port devices, mirrors, birefringent crystals, etc., the optical demultiplexer 10 separates the multiplexed signal 14 into its constituent parts. Each of the individual wavelengths, each representing a separate signal on a communication channel, is then output to one of output ports 16a-16d. 
A multiplexer functions in the inverse manner as the demultiplexer. Multiplexers can often be constructed from demultiplexers simply by using the output ports 16 as input ports and the input port 12 as an output port.
An optical device that combines the functionality of a demultiplexer and a multiplexer is known as a mux/demux. An exemplary mux/demux is shown in FIG. 2 and designated generally as 20. The mux/demux 20 has a multiplexed input port 22 that accepts as its input a multiplexed transmission 14. The multiplexed transmission 14 is separated into its constituent parts and output to demultiplexed output ports 24. In a multiplexing operation, demultiplexed input ports 26 accept as their input individual signals, with each signal being encoded on a different optical wavelength. The individual signals are combined into a multiplexed transmission 15 and output to the multiplexed output 28.
An OADM is a component designed to extract an individual signal from a multiplexed transmission while allowing the remaining signals on the multiplexed transmission to pass through. The OADM also has an add port that can be used to remix the extracted signal with the multiplexed transmission or to transmit other data onto the fiber-optic network. An example of an OADM is shown in FIG. 3 and designated generally as 30. The OADM 30 is designed for bi-directional data communication. In optical networks, to distinguish the direction of data travel, the directions are referred to as east and west directions. In FIG. 3, data that travels in an easterly direction travels to the right of the OADM 30. Data the travels in a westerly direction travels to the left of the OADM 30.
Illustrating the functionality of the OADM 30, a multiplexed transmission 14 is input into the west input port 32. The OADM 30 is designed for a specific wavelength or, more precisely, a band of wavelengths. For example, if the particular multiplexed transmission has optical signals over four wavelength channels, including a 1510 nanometer wavelength, a 1530 nanometer wavelength, a 1550 manometer wavelength, and a 1570 nanometer wavelength, and the OADM 30 is designed to extract signals transmitted on the 1550 nanometer wavelength, the OADM may in fact extract any signal within an approximately 12 nanometer bandwidth centered about the 1550 nanometer wavelength. As such, any wavelength between 1544 and 1556 nm is extracted by the OADM 30. In the present example, an individual signal 34 is extracted from the multiplexed transmission 14 and output to a device existing on the network, such as a network node 36, through the west drop port 38.
All other wavelengths remaining on the multiplexed transmission continue through the OADM 30 and exit through an east output port 40, where they may continue to propagate on the fiber-optic network. If the OADM is a bidirectional module, such as OADM 30, a multiplexed transmission traveling in a westerly direction enters the OADM 30 at the east input port 48, drops the particular signal through the east drop port 47, adds a signal through the west add port 44, and propagates the remaining wavelengths through the west output port 49.
The network node 36 has two transceiver modules 42. In one embodiment, the transceiver modules may be GigaBit Interface Components (GBICs). The transceiver modules 42 have an input port for accepting optical signals so that the signals can be converted to a data signal useful by the network node 36, and output ports for generating optical signals from the network node 36 so that data from the network node 36 may be propagated on the fiber-optic network. Optical signals from the network node 36 may be propagated onto the fiber-optic network such that they travel in a westerly direction by inputting the signals into the west add port 44 or propagated to the fiber-optic network, such that they travel in an easterly direction by inputting the signal signals into the east add port 45. By using an OADM that is bidirectional, redundancy may be added to the optical fiber network to provide for such contingencies as broken fibers in one of the directions. Optical add/drop multiplexers, such as OADM 30, are generally passive devices and are constructed using thin-film three-port devices, fused fiber devices, or other passive components.
WDM systems with dual fibers typically use unidirectional signal transmission on each fiber to accommodate the traffic in each direction. Such dual line systems can provide an advantage in providing systems with a system redundancy. For example, FIGS. 4A and 4B depict a double line ring structure 50. The ring includes a multiplexer/demultiplexer (“mux/demux”) module 52, a series of optical add/drop multiplexers (“OADMs”) 54, 56, 58, 60, and double lines, or fibers, 62, 64. In the event that the double lines 62, 64 are broken, as depicted at break 66 in FIG. 4B, each of OADMs 58 and 60 can take the signal traveling a first direction down one fiber and redirect the signal down the parallel fiber in the opposite direction, as depicted by arrows 68, 70. This process is depicted in greater detail in FIG. 5, where it seen that, by way of example, a 1470 wavelength signal can be added or dropped from a first line 62 and added or dropped to a second line 64 so that the transmission in the loop is maintained.
In addition, in dual line systems, should one fiber become broken, the other fiber can provide a redundancy that can be used to restore or redirect data transmission as desired.
The main disadvantage in dual line systems is the cost in creating, maintaining, and purchasing or leasing a dual line system. For example, businesses having multiple campuses often rent lines for communication across external networks. The cost of renting the lines is set in part by the number of fibers and the length over which they travel. By way of example, a forty kilometer dual line fiber rental at one hundred dollars per month per kilometer would cost eight thousand dollars per month.
Accordingly, there is a continuing need for improved and less expensive methods and devices for decreasing the cost of data transmission without sacrificing system safeguards.